/*
 * @(#)Collection.java	1.49 04/06/28
 *
 * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package java.util;

import java.util.function.Predicate;

/**
 * The root interface in the <i>collection hierarchy</i>. A collection
 * represents a group of objects, known as its <i>elements</i>. Some collections
 * allow duplicate elements and others do not. Some are ordered and others
 * unordered. The JDK does not provide any <i>direct</i>
 * implementations of this interface: it provides implementations of more
 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface is
 * typically used to pass collections around and manipulate them where maximum
 * generality is desired.
 *
 * <p>
 * <i>Bags</i> or <i>multisets</i> (unordered collections that may contain
 * duplicate elements) should implement this interface directly.
 *
 * <p>
 * All general-purpose <tt>Collection</tt> implementation classes (which
 * typically implement <tt>Collection</tt> indirectly through one of its
 * subinterfaces) should provide two "standard" constructors: a void (no
 * arguments) constructor, which creates an empty collection, and a constructor
 * with a single argument of type <tt>Collection</tt>, which creates a new
 * collection with the same elements as its argument. In effect, the latter
 * constructor allows the user to copy any collection, producing an equivalent
 * collection of the desired implementation type. There is no way to enforce
 * this convention (as interfaces cannot contain constructors) but all of the
 * general-purpose <tt>Collection</tt>
 * implementations in the Java platform libraries comply.
 *
 * <p>
 * The "destructive" methods contained in this interface, that is, the methods
 * that modify the collection on which they operate, are specified to throw
 * <tt>UnsupportedOperationException</tt> if this collection does not support
 * the operation. If this is the case, these methods may, but are not required
 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
 * have no effect on the collection. For example, invoking the
 * {@link #addAll(Collection)} method on an unmodifiable collection may, but is
 * not required to, throw the exception if the collection to be added is empty.
 *
 * <p>
 * Some collection implementations have restrictions on the elements that they
 * may contain. For example, some implementations prohibit null elements, and
 * some have restrictions on the types of their elements. Attempting to add an
 * ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting to
 * query the presence of an ineligible element may throw an exception, or it may
 * simply return false; some implementations will exhibit the former behavior
 * and some will exhibit the latter. More generally, attempting an operation on
 * an ineligible element whose completion would not result in the insertion of
 * an ineligible element into the collection may throw an exception or it may
 * succeed, at the option of the implementation. Such exceptions are marked as
 * "optional" in the specification for this interface.
 *
 * <p>
 * This interface is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * <p>
 * Many methods in Collections Framework interfaces are defined in terms of the
 * {@link Object#equals(Object) equals} method. For example, the specification
 * for the {@link #contains(Object) contains(Object o)} method says: "returns
 * <tt>true</tt> if and only if this collection contains at least one element
 * <tt>e</tt> such that
 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
 * invoked for any element <tt>e</tt>. Implementations are free to implement
 * optimizations whereby the <tt>equals</tt> invocation is avoided, for example,
 * by first comparing the hash codes of the two elements. (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.) More generally, implementations of the
 * various Collections Framework interfaces are free to take advantage of the
 * specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @version 1.49, 06/28/04
 * @see	Set
 * @see	List
 * @see	Map
 * @see	SortedSet
 * @see	SortedMap
 * @see	HashSet
 * @see	TreeSet
 * @see	ArrayList
 * @see	LinkedList
 * @see	Vector
 * @see Collections
 * @see	Arrays
 * @see	AbstractCollection
 * @since 1.2
 */
public interface Collection<E> extends java.lang.Iterable<E> {
    // Query Operations

    /**
     * Returns the number of elements in this collection. If this collection
     * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of elements in this collection
     */
    int size();

    /**
     * Returns <tt>true</tt> if this collection contains no elements.
     *
     * @return <tt>true</tt> if this collection contains no elements
     */
    boolean isEmpty();

    /**
     * Returns <tt>true</tt> if this collection contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this collection
     * contains at least one element <tt>e</tt> such that
     * <tt>(o==null ? e==null : o.equals(e))</tt>.
     *
     * @param o element whose presence in this collection is to be tested.
     * @return <tt>true</tt> if this collection contains the specified element
     * @throws ClassCastException if the type of the specified element is
     * incompatible with this collection (optional).
     * @throws NullPointerException if the specified element is null and this
     * collection does not support null elements (optional).
     */
    boolean contains(Object o);

    /**
     * Returns an iterator over the elements in this collection. There are no
     * guarantees concerning the order in which the elements are returned
     * (unless this collection is an instance of some class that provides a
     * guarantee).
     *
     * @return an <tt>Iterator</tt> over the elements in this collection
     */
    Iterator<E> iterator();

    /**
     * Returns an array containing all of the elements in this collection. If
     * the collection makes any guarantees as to what order its elements are
     * returned by its iterator, this method must return the elements in the
     * same order.<p>
     *
     * The returned array will be "safe" in that no references to it are
     * maintained by this collection. (In other words, this method must allocate
     * a new array even if this collection is backed by an array). The caller is
     * thus free to modify the returned array.<p>
     *
     * This method acts as bridge between array-based and collection-based APIs.
     *
     * @return an array containing all of the elements in this collection
     */
    Object[] toArray();

    /**
     * Returns an array containing all of the elements in this collection; the
     * runtime type of the returned array is that of the specified array. If the
     * collection fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this collection.<p>
     *
     * If this collection fits in the specified array with room to spare (i.e.,
     * the array has more elements than this collection), the element in the
     * array immediately following the end of the collection is set to
     * <tt>null</tt>. This is useful in determining the length of this
     * collection <i>only</i> if the caller knows that this collection does not
     * contain any <tt>null</tt> elements.)
     * <p>
     *
     * If this collection makes any guarantees as to what order its elements are
     * returned by its iterator, this method must return the elements in the
     * same order.<p>
     *
     * Like the <tt>toArray</tt> method, this method acts as bridge between
     * array-based and collection-based APIs. Further, this method allows
     * precise control over the runtime type of the output array, and may, under
     * certain circumstances, be used to save allocation costs<p>
     *
     * Suppose <tt>l</tt> is a <tt>List</tt> known to contain only strings. The
     * following code can be used to dump the list into a newly allocated array
     * of <tt>String</tt>:
     *
     * <pre>
     *     String[] x = (String[]) v.toArray(new String[0]);
     * </pre><p>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of this collection are to be
     * stored, if it is big enough; otherwise, a new array of the same runtime
     * type is allocated for this purpose.
     * @return an array containing the elements of this collection
     *
     * @throws ArrayStoreException the runtime type of the specified array is
     * not a supertype of the runtime type of every element in this collection.
     * @throws NullPointerException if the specified array is <tt>null</tt>.
     */
    <T> T[] toArray(T[] a);

    // Modification Operations
    /**
     * Ensures that this collection contains the specified element (optional
     * operation). Returns <tt>true</tt> if this collection changed as a result
     * of the call. (Returns <tt>false</tt> if this collection does not permit
     * duplicates and already contains the specified element.)
     * <p>
     *
     * Collections that support this operation may place limitations on what
     * elements may be added to this collection. In particular, some collections
     * will refuse to add <tt>null</tt> elements, and others will impose
     * restrictions on the type of elements that may be added. Collection
     * classes should clearly specify in their documentation any restrictions on
     * what elements may be added.<p>
     *
     * If a collection refuses to add a particular element for any reason other
     * than that it already contains the element, it <i>must</i> throw an
     * exception (rather than returning <tt>false</tt>). This preserves the
     * invariant that a collection always contains the specified element after
     * this call returns.
     *
     * @param o element whose presence in this collection is to be ensured.
     * @return <tt>true</tt> if this collection changed as a result of the call
     *
     * @throws UnsupportedOperationException <tt>add</tt> is not supported by
     * this collection.
     * @throws ClassCastException class of the specified element prevents it
     * from being added to this collection.
     * @throws NullPointerException if the specified element is null and this
     * collection does not support null elements.
     * @throws IllegalArgumentException some aspect of this element prevents it
     * from being added to this collection.
     */
    boolean add(E o);

    /**
     * Removes a single instance of the specified element from this collection,
     * if it is present (optional operation). More formally, removes an element
     * <tt>e</tt> such that <tt>(o==null ? e==null : o.equals(e))</tt>, if this
     * collection contains one or more such elements. Returns true if this
     * collection contained the specified element (or equivalently, if this
     * collection changed as a result of the call).
     *
     * @param o element to be removed from this collection, if present.
     * @return <tt>true</tt> if this collection changed as a result of the call
     *
     * @throws ClassCastException if the type of the specified element is
     * incompatible with this collection (optional).
     * @throws NullPointerException if the specified element is null and this
     * collection does not support null elements (optional).
     * @throws UnsupportedOperationException remove is not supported by this
     * collection.
     */
    boolean remove(Object o);

    // Bulk Operations
    /**
     * Returns <tt>true</tt> if this collection contains all of the elements in
     * the specified collection.
     *
     * @param c collection to be checked for containment in this collection.
     * @return <tt>true</tt> if this collection contains all of the elements in
     * the specified collection
     * @throws ClassCastException if the types of one or more elements in the
     * specified collection are incompatible with this collection (optional).
     * @throws NullPointerException if the specified collection contains one or
     * more null elements and this collection does not support null elements
     * (optional).
     * @throws NullPointerException if the specified collection is
     * <tt>null</tt>.
     * @see #contains(Object)
     */
    boolean containsAll(Collection<?> c);

    /**
     * Adds all of the elements in the specified collection to this collection
     * (optional operation). The behavior of this operation is undefined if the
     * specified collection is modified while the operation is in progress.
     * (This implies that the behavior of this call is undefined if the
     * specified collection is this collection, and this collection is
     * nonempty.)
     *
     * @param c elements to be inserted into this collection.
     * @return <tt>true</tt> if this collection changed as a result of the call
     *
     * @throws UnsupportedOperationException if this collection does not support
     * the <tt>addAll</tt> method.
     * @throws ClassCastException if the class of an element of the specified
     * collection prevents it from being added to this collection.
     * @throws NullPointerException if the specified collection contains one or
     * more null elements and this collection does not support null elements, or
     * if the specified collection is <tt>null</tt>.
     * @throws IllegalArgumentException some aspect of an element of the
     * specified collection prevents it from being added to this collection.
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> c);

    /**
     *
     * Removes all this collection's elements that are also contained in the
     * specified collection (optional operation). After this call returns, this
     * collection will contain no elements in common with the specified
     * collection.
     *
     * @param c elements to be removed from this collection.
     * @return <tt>true</tt> if this collection changed as a result of the call
     *
     * @throws UnsupportedOperationException if the <tt>removeAll</tt> method is
     * not supported by this collection.
     * @throws ClassCastException if the types of one or more elements in this
     * collection are incompatible with the specified collection (optional).
     * @throws NullPointerException if this collection contains one or more null
     * elements and the specified collection does not support null elements
     * (optional).
     * @throws NullPointerException if the specified collection is
     * <tt>null</tt>.
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    default boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        boolean removed = false;
        final Iterator<E> each = iterator();
        while (each.hasNext()) {
            if (filter.test(each.next())) {
                each.remove();
                removed = true;
            }
        }
        return removed;
    }

    /**
     * Retains only the elements in this collection that are contained in the
     * specified collection (optional operation). In other words, removes from
     * this collection all of its elements that are not contained in the
     * specified collection.
     *
     * @param c elements to be retained in this collection.
     * @return <tt>true</tt> if this collection changed as a result of the call
     *
     * @throws UnsupportedOperationException if the <tt>retainAll</tt> method is
     * not supported by this Collection.
     * @throws ClassCastException if the types of one or more elements in this
     * collection are incompatible with the specified collection (optional).
     * @throws NullPointerException if this collection contains one or more null
     * elements and the specified collection does not support null elements
     * (optional).
     * @throws NullPointerException if the specified collection is
     * <tt>null</tt>.
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean retainAll(Collection<?> c);

    /**
     * Removes all of the elements from this collection (optional operation).
     * This collection will be empty after this method returns unless it throws
     * an exception.
     *
     * @throws UnsupportedOperationException if the <tt>clear</tt> method is not
     * supported by this collection.
     */
    void clear();

    // Comparison and hashing
    /**
     * Compares the specified object with this collection for equality.
     * <p>
     *
     * While the <tt>Collection</tt> interface adds no stipulations to the
     * general contract for the <tt>Object.equals</tt>, programmers who
     * implement the <tt>Collection</tt> interface "directly" (in other words,
     * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
     * or a <tt>List</tt>) must exercise care if they choose to override the
     * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
     * course of action is to rely on <tt>Object</tt>'s implementation, but the
     * implementer may wish to implement a "value comparison" in place of the
     * default "reference comparison." (The <tt>List</tt> and
     * <tt>Set</tt> interfaces mandate such value comparisons.)
     * <p>
     *
     * The general contract for the <tt>Object.equals</tt> method states that
     * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
     * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
     * and <tt>Set.equals</tt> state that lists are only equal to other lists,
     * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
     * collection class that implements neither the <tt>List</tt> nor
     * <tt>Set</tt> interface must return <tt>false</tt> when this collection is
     * compared to any list or set. (By the same logic, it is not possible to
     * write a class that correctly implements both the <tt>Set</tt> and
     * <tt>List</tt> interfaces.)
     *
     * @param o Object to be compared for equality with this collection.
     * @return <tt>true</tt> if the specified object is equal to this collection
     *
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     * @see List#equals(Object)
     */
    boolean equals(Object o);

    /**
     * Returns the hash code value for this collection. While the
     * <tt>Collection</tt> interface adds no stipulations to the general
     * contract for the <tt>Object.hashCode</tt> method, programmers should take
     * note that any class that overrides the <tt>Object.equals</tt>
     * method must also override the <tt>Object.hashCode</tt> method in order to
     * satisfy the general contract for the <tt>Object.hashCode</tt>method. In
     * particular, <tt>c1.equals(c2)</tt> implies that
     * <tt>c1.hashCode()==c2.hashCode()</tt>.
     *
     * @return the hash code value for this collection
     *
     * @see Object#hashCode()
     * @see Object#equals(Object)
     */
    int hashCode();
}
